For me, and for many others, champions with varying modifiers will be a something that keeps the game challenging for a good time after release.
So here is our think-tank of possible modifiers that hopefully will kick our butts many times over :cloud9:
(http://www.diablowiki.net/Boss_modifiers)<----- Official modifiers

Warded: Provides champion with an aura. Named aura nerfs/absorbs any attack launched at the champion from the outside of the aura

Thorns: Champion curses his enemy/-ies, making them take damage if they move (by move I mean walk/run)

Cruel: The champion has an increased chance to score critical hits (leathal when combined with elemental damage)

Juggernaut: The champion can, when theatened, activate a shield that absorbs all incoming damage for a few seconds

Devourer: Every time someone (monster/minion/character) dies in the vicininy of a monster gifted with this modifier named monster gains a permanent buff to it's stats (based on the power of killed monster/minion/character)

Destroyer: Slows down the monster's attack speed, but boosts it's damage output per attack many times over

Mirror Armor: Periodically casts Mirror Armor on him/herself, which returns damage back to ranged attackers (Thorns Aura for casters/ranged!). Mirror Armor is not up 100% of the time.

Invisibility: Periodically casts Invisibility on him/herself or on a minion. Invisibility is not up 100% of the time.

Cursed: Hits/missiles can cast curses on players. A monster will only have one kind of curse at a time, but the list of potential curses has grown, and you won't know what kind of curse the monster has until it hits you. Amp Damage, Slow, Lower Resist, Weaken, Hallucinate (causes the player to see a massive number of harmless Doppelgangers. The player cannot tell the difference between the Doppelgangers and real monsters).

Copier - Monster is granted one of your abilities to use at will.
Shapeshifter - Can change into another monster type
MSLE - MSLE
Death Pact - Can sacrifice a minion or another monster to regain life
Mind control - Can take control of your follower or a summon (never a character)
Thief - Makes you drop your potions (this time it isn't as buggy)
Globe Trotter - It moves over to health globes and tries to take them from you.

Possession: character movement drastically slowed (fighting the possession), the skill active at time becomes unable to be used with mana respectively drained a constant rate throughout the duration of possession.

...interesting component as possession and spirit containment is a major reoccurring theme in the story. This could occur when an act boss has 25% or lower health indicating that it now has an agenda (effectively its plan b) to posses your character as it may not survive the battle. This possession would be short lived (a defined amount of seconds) to simply serve as an annoying hurdle to overcome.

Volcanic = Striking the creature with short ranged attacks causes fire dot retaliation. Striking the creature with longer range attacks causes it to counterattack by hurling volcanic orbs, which explode into a number of smaller orbs that spread out and then explode into shrapnel.
Glacial = Simply being near the creature slows your movement, attack speed, and casting speed. Long ranged attacks provoke ice prison counterattacks that your attacks cannot pass through but can destroy, and enemy attacks can pass through.
Stormcharged = Has a short range D2 Thunderstorm like effect. Moves much faster than normal and if melee will seek to quickly close to ranged attackers. Ranged attacks have a higher than normal chance to miss (be struck down by stray lightning bolts).

Basalt - Boss spawns as an unmoving stone statue. This renders the boss unable to use physical attacks, but also immune to physical attacks. The monster gains the ability to conjure fireballs along with any magical abilities it may have already had.

Charging Bull - Boss has its running speed increased 3 times, but loses its ability to stop or turn without colliding with an obstacle. If a player is hit by the charge they take significant damage and are knocked back and stunned. If the boss collides with the map then it gets stunned. If the boss charges towards a cliff that would normally have an invisible wall preventing the player from crossing the boss ignores the wall and charges off and dies in the fall but drops its loot at the edge.

Necromancy - When the boss is killed its skeleton steps out of its corpse and continues the fight with the same stats as the original. If the boss lost an appendage due to some sort of critical death animation then that appendage remains missing from the skeleton.

Shade of the Lightless Void - Boss absorbs all light in a large circle around it leaving pure black. The player cannot see into the circle nor anything beyond it but the player can still see outside the circle on the side of their approach if they step inside as the light is pulled inwards. The circle is not centered around the boss but rather the boss can be lurking anywhere inside of the circle.

Strange Attractor - All nearby movable objects (jars, arrows, barrels, benches, etc.) are periodically attracted to orbit the boss and deflect attacks and then repelled to fly out radially and damage friend and foe alike.

Acidic: Mob's attacks deal additional acid damage, mob's attacks also ignore a % of targets armor/damage reduction. Each attack against the mob has a chance of dealing X-Y acid damage to all players within Z yards.

Combine that with extra fast and people will run away from them in droves... Permanent nerfs is one of the things that will cause a great amount of rage quits/Character trashings
Make the nerf disappear when you complete hell and it will be OK